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Axiomatic cellular boundaries are integral incidence matrices with coefficients
Statement
For a CW pair , an ordinary theory with coefficient , and chosen cell orientations, the complex is canonically Its differential is the integral incidence matrix acting on . In degree one the entries are signed terminal-minus-initial endpoints. The direct-sum matrices have finite support in each column.
Facts & Assumptions
Given: The objects and hypotheses in the statement above.
For a CW pair and an ordinary theory with coefficient , put and for . Set Each is the direct sum of copies of indexed by the relative -cells. Define and for let be the triple boundary to followed by its map to . Then , naturally for cellular maps of CW pairs. (Any ordinary homology theory has a cellular chain complex on a cw pair)
Let and be continuous. If on is multiplication by , then on for every ordinary theory is . The identifications use the same oriented sphere generator; for use the difference of the two point classes. (Sphere endomorphisms act by the same integer in every ordinary theory)
For and oriented cells and , collapse the complement of in and compose the attaching map of with the resulting quotient to . Its induced endomorphism of oriented is multiplication by a unique integer, denoted . For , orient the characteristic interval of from to and, for a vertex , set Thus an oriented edge contributes its terminal vertex minus its initial vertex, and a loop with both endpoints at one vertex has incidence number zero there. (Incidence number of two CW cells)
Let be a CW complex. For , in the integral cellular chain groups with the chosen cell orientations, (Cellular boundary is the incidence degree matrix)
Proof
By F1 the chain groups are direct sums of copies of on relative cells. For a source -cell and target -cell with , project the boundary homomorphism onto the target summand. Naturality of the characteristic disk pair identifies this component with the attaching map followed by collapse onto the target cell sphere and the natural disk boundary identifications. The corresponding integer for integral singular homology is precisely the incidence number of F3 and F4.
F2 says that this same sphere endomorphism acts on coefficient by that integer times the identity. For , the boundary of the oriented interval is at its two ends, so an edge contributes at the terminal vertex and at the initial vertex. If the endpoints coincide they cancel; endpoints in vanish in the relative complex.
Each characteristic boundary has image in the finite union of closed cells supplied by closure finiteness, so only finitely many target cells can contribute to its column. Thus these components define a map of direct sums. At degree zero the outgoing differential is zero. The bases and component calculations identify the entire complex with the displayed tensor complex, for arbitrary , including and pairs with no relative cells.
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Sources
- May, A Concise Course in Algebraic Topology, 15§2, first theorem and arbitrary-coefficient paragraph p.119 (standard reference, not scraped)